Before designing a multilayer PCB board, the designer needs to first determine the board structure to be used according to the size of the circuit, the size of the board, and the requirements of electromagnetic compatibility (EMC), that is, whether to decide to use 4 layers, 6 layers, or More layers of boards. After determining the number of layers, determine where the inner layers are placed and how different signals are distributed across the layers. This is the choice of multilayer PCB laminate structure.
The laminated structure is an important factor affecting the EMC performance of the PCB and an important means of suppressing electromagnetic interference. This article describes the related content of the multilayer PCB laminate structure.
After the power supply, the number of layers of the ground, and the number of signal layers are determined, the relative arrangement between them is a topic that every PCB engineer cannot avoid;
The general principle of layer layout:
1. Determining the laminated structure of a multilayer PCB board requires consideration of many factors. In terms of wiring, the more layers, the more convenient the wiring, but the cost and difficulty of board making will also increase. For the manufacturer, the symmetry of the laminated structure is the focus of PCB board manufacturing, so the choice of the number of layers needs to consider all aspects of the need to achieve the best balance. For experienced designers, after the pre-layout of components is completed, the PCB bottlenecks will be analyzed. Combine other EDA tools to analyze the wiring density of the board; then combine the number and type of signal lines such as differential lines and sensitive signal lines with special wiring requirements to determine the number of layers of the signal layer; then according to the type of power supply, isolation and anti-interference The requirements to determine the number of inner layers. Thus, the number of layers of the entire board is basically determined.
2. The underside of the component surface (the second layer) is the ground plane, providing the device shielding layer and providing a reference plane for the top layer wiring; the sensitive signal layer should be adjacent to an internal electrical layer (internal power supply/ground layer), utilizing the large internal power layer A copper film provides shielding for the signal layer. The high speed signal transmission layer in the circuit should be the signal intermediate layer and sandwiched between the two inner layers. The copper films of the two inner layers can provide electromagnetic shielding for high-speed signal transmission, and can also effectively limit the radiation of high-speed signals between the two inner layers without external interference.
3. All signal layers are as close as possible to the ground plane;
4. Try to avoid the direct proximity of the two signal layers; crosstalk is easily introduced between adjacent signal layers, resulting in failure of the circuit function. Adding a ground plane between the two signal layers can effectively avoid crosstalk.
5. The main power source is adjacent to it as far as possible;
6, taking into account the symmetry of the laminated structure.
7. For the layer layout of the mother board, it is difficult to control the parallel long distance wiring for the existing motherboard. For the board level operating frequency above 50MHZ (refer to the case below 50MHZ, appropriate relaxation), the recommended layout principle:
The component surface and the welding surface are complete ground planes (shield);
No adjacent parallel wiring layers;
All signal layers are as close as possible to the ground plane;
The key signals are adjacent to the formation and do not span the partition.
Note: When setting the layer of the specific PCB, it is necessary to flexibly master the above principles. Based on the above principles, according to the requirements of the actual board, such as whether a critical wiring layer, power supply, ground plane division, etc. are required. To determine the layout of the layers, avoid moving the hard cover, or hold it aside.
8. Multiple grounded internal layers can effectively reduce the ground impedance. For example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common mode interference.
Commonly used laminate structure:
4-layer board
The following is an example of how to optimize the arrangement and combination of various laminated structures by way of an example of a 4-layer board.
For the commonly used 4-layer boards, there are several stacking methods (from top to bottom).
(1) Siganl_1 (Top), GND (Inner_1), POWER (Inner_2), Siganl_2 (Bottom).
(2) Siganl_1 (Top), POWER (Inner_1), GND (Inner_2), Siganl_2 (Bottom).
(3) POWER (Top), Siganl_1 (Inner_1), GND (Inner_2), Siganl_2 (Bottom).
Obviously, the lack of effective coupling between the power layer and the ground plane of scheme 3 should not be adopted.
So how should Option 1 and Option 2 be chosen?
In general, the designer will choose option 1 as the structure of the 4-layer board. The reason for the selection is not that Option 2 cannot be used, but that the general PCB board only places components on the top layer, so it is more appropriate to adopt Option 1.
However, when components need to be placed on both the top and bottom layers, and the thickness of the medium between the internal power layer and the ground layer is large, and the coupling is not good, it is necessary to consider which layer has fewer signal lines. For Option 1, there are fewer signal lines on the bottom layer, and a large area of ​​copper film can be used to couple with the POWER layer. Conversely, if the components are mainly placed on the bottom layer, Option 2 should be used to make the board.
If a stacked structure is used, the power layer and the ground layer itself are already coupled. Considering the requirement of symmetry, scheme 1 is generally adopted.
6-layer board
After the analysis of the laminated structure of the 4-layer board is completed, the arrangement and combination method of the 6-layer board laminated structure and the preferred method are explained below by way of an example of a 6-layer board combination.
(1) Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), Siganl_3 (Inner_3), POWER (Inner_4), Siganl_4 (Bottom).
Scheme 1 uses four layers of signal layers and two layers of internal power/ground layers. It has more signal layers, which is good for wiring between components. However, the defects of this scheme are also obvious, which are manifested in the following two aspects:
1 The power and ground planes are separated far apart and are not fully coupled.
2 The signal layer Siganl_2 (Inner_2) and Siganl_3 (Inner_3) are directly adjacent, and the signal isolation is not good, and crosstalk is easy to occur.
(2) Siganl_1 (Top), Siganl_2 (Inner_1), POWER (Inner_2), GND (Inner_3), Siganl_3 (Inner_4), Siganl_4 (Bottom).
Scheme 2 Compared with scheme 1, the power layer and the ground layer are fully coupled, which has certain advantages over scheme 1, but
Siganl_1 (Top) and Siganl_2 (Inner_1) and Siganl_3 (Inner_4) and Siganl_4 (Bottom) signal layers are directly adjacent, signal isolation is not good, and the problem of crosstalk is not solved.
(3) Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), POWER (Inner_3), GND (Inner_4), Siganl_3 (Bottom).
Compared with schemes 1 and 2, scheme 3 reduces one signal layer and adds one inner layer. Although the available wiring level is reduced, the scheme solves the defects common to schemes 1 and 2.
1 The power and ground planes are tightly coupled.
2 Each signal layer is directly adjacent to the inner layer, and is effectively isolated from other signal layers, which is less prone to crosstalk.
3 Siganl_2 (Inner_2) and two internal layers GND (Inner_1) and POWER (Inner_3) can be used to transmit high-speed signals. The two inner layers can effectively shield the external interference to the Siganl_2 (Inner_2) layer and the interference of the Siganl_2 (Inner_2) to the outside world.
In all aspects, Option 3 is clearly the most optimized one. At the same time, Option 3 is also a common stacking structure for 6-layer boards. Through the analysis of the above two examples, I believe that readers have already had a certain understanding of the cascading structure, but in some cases, a certain solution can not meet all the requirements, which requires consideration of the priority of each design principle. Unfortunately, because the board design of the board is closely related to the characteristics of the actual circuit, the anti-interference performance and design focus of different circuits are different, so in fact these principles have no certain priority for reference. However, it can be confirmed that design principle 2 (the internal power supply layer and the ground layer should be tightly coupled) needs to be satisfied first in design, and if high-speed signals need to be transmitted in the circuit, then design principle 3 (high-speed signal transmission layer in the circuit) It should be the middle layer of the signal and sandwiched between the two inner layers) must be satisfied.
10-layer board
PCB typical 10-layer board design
The general common wiring sequence is TOP--GND---Signal layer---Power layer---GND---Signal layer---Power layer---Signal layer---GND---BOTTOM
The wiring order itself is not necessarily fixed, but there are some standards and principles to constrain: for example, the top layer and the adjacent layer of the bottom are GND to ensure the EMC characteristics of the board; for example, the signal layer is preferably used as a reference for the GND layer. Plane; the power supply used by the entire board is preferentially paved with a piece of copper; the interference is high, the speed is high, and the inner layer is preferably hopped.
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